This invention relates to anode structures, and more particularly, to impressed current sacrificial anodes for use in cathodic protection systems.
Use of sacrificial anodes for cathodic protection is a well established practice. Special anode designs have even been developed to compensate for the conditions which may be encountered in such systems. Thus in U.S. Pat. Nos. 3,043,765 and 3,239,443, both to Bryan et al and both assigned to the present assignee, there is disclosed high silicon iron anodes having unique connections which are protected from corrosive attack.
While these end structures are quite satisfactory for mild service conditions, in deep groundbed applications or in sea water service there is the need for a completely encapsulated moisture seal. An anode structure filling this need is disclosed in Sumner U.S. Pat. No. 3,471,395, assigned to the assignee of the present invention. There a heat shrinkable fluorocarbon resin sheath is used to form an encapsulated end seal.
Anodes of this design have proved excellent for moisture sealing, but may face additional problems in service. All anodes tend to discharge a high percentage of their total current from the extreme ends of the anode, resulting in a much higher corrosion rate at these locations. This high current density at the end is called current crowding or end effect and, depending on the severity of service, metal consumption at this area can be great enough to cause anode separation in a short period of time, i.e., undercutting takes place at the end adjacent the end cap.
The insulating caps described in U.S. Pat. No. 3,471,395 protect the anode surface immediately under the cap, but this merely moves the "end" to the adjacent unprotected metal. Rapid corrosion at this point will then progress across the diameter of the anode and could even cut all the way through, allowing a large percentage of the anode weight to separate and fall away. Other end cap arrangements are shown in U.S. Pat. Nos. 3,046,213 to Bender and 2,816,069 to Andrus, but would for the aforesaid reasons suffer the same fate under the conditions described.
Localized rapid consumption of sacrifical anodes has been a problem in other areas too. For that reason, Anderson in U.S. Pat. No. 3,010,891 discloses a means of replacing the consumed portions of a trailing anode on ocean-going vessels. Thus, Anderson feeds a wire anode through a tube in such a manner that as the wire is consumed, it is replaced by a new length. Another anode structure with replaceable segments is shown in Krenzke, U.S. Pat. No. 3,016,343.
While replaceable segments is one solution, it is not always possible to effect such a replacement. For that reason, a more permanent installation is required for deep groundbed and sea water use. In that regard it is known to use steel pipes as anodes in deep groundbeds. The connection is welded to the pipe and is coated or extends out of the ground. The pipe is installed in sections with each section welded to the other for 200-300 feet depending on the hole depth. A coating strip running the full length of the pipe protects the pipe immediately beneath the strip thereby providing a path for current as the metal is being consumed during operation. This is a very costly setup. Besides, it does not eliminate the problems of current crowding and metal consumption at the point of the electrical connection.
Encapsulated rectifier anodes have also been used. In those structures the wire is welded to the inside of a pipe reducer. However, this would be impossible to do in a tubular anode. Likewise, anodes of this and other solid form suffer from another problem associated with deep groundbed or sea water service in that there is formation of a gas at the surface of the anode. This condition tends to occur because of the high current dissipation per square foot of anode surface. Gas bubbles insulate the surface and, therefore, inhibit the discharge of current.
Accordingly, there exists a need for an anode structure which will minimize the problem of gas blocking while at the same time offering longer service time with no current crowding and resultant undercutting.